Discharge lamps are classified roughly into low-pressure discharge lamps and high-pressure discharge lamps. Low-pressure discharge lamps include arc discharge-type discharge lamps, for example, general lightings, special lightings for use, for example, in roads and tunnels, coating material curing apparatuses, UV (ultraviolet) curing apparatuses, sterilizers, and light cleaning apparatuses, for example, for semiconductors. High-pressure discharge lamps include apparatuses for water supply and sewerage, general lightings, exterior lightings, for example, in stadiums, UV curing apparatuses, exposure devices, for example, for semiconductors and printed boards, wafer inspection apparatuses, high-pressure mercury lamps, for example, for projectors, metal halide lamps, ultrahigh-pressure mercury lamps, xenon lamps, and sodium lamps. Thus, discharge lamps are used for various apparatuses such as lighting apparatuses and production apparatuses.
Tungsten alloys containing thorium oxide (ThO2) have hitherto been used in cathode components for discharge lamps. Japanese Patent Application Laid-Open No. 226935/2002 discloses a thorium-containing tungsten alloy that has been improved in resistance to deformation by finely dispersing thorium and a thorium compound in a mean grain size of not more than 0.3 μm.